Probably not the best reason to run it in open air, howevever reduced cost of tunnelin is.
I generally agree with Gweed on LRT conversion though. However, it would still be a relatively lower speed line and not appropriate for crosstown trips. For now, the 407 bus corridor will work fine for crosstown trips.
Question about the Finch Hydro ROW. Would it still be possible to run a heavy rail line along the corridor? However, once volume warrants it, I can easily see a new rail line constructed from Oakville to Pickering following the 403, Richview Expressway Corridor west of the 427, and the Finch Hydro ROW hitting stops at Square One, Renforth Gateway, Etobicoke North, Keele/Finch, and Yonge/Finch. This would be a far future project on the timeline of 30+ years, but if the corridor can be protected for today, it would save a lot of headaches in the future.